It may also be a JB 4.2 issue and not the phone. I've seen posts elsewhere about this on the nexus 4 and the nexus7. Also since updating to 4.2 my nexus 7 does it occasionally where it never did before.

Had mine for about 1 1/2 weeks on 4.2 and not a single reboot. I'm thinking it has to be an app causing it. Probably something that is running in the background which is why you haven't noticed what is causing it.

Ive had 3 reboots(shut downs?) so far since Tuesday when I got the phone. I am running GoLauncher so maybe thats it? I have found a few apps so far that dont play well with 4.2.

Doesnt really bother me at this point in time. 2 of the three times it happened when I was sleeping and it was 2 of the first times I actually slept through the night without being woken up by my phone.